Explain why action-reaction forces do not cancel each other out, resulting in zero net force.

Action and reaction are acting on two different objects. A net force can only be calculated from forces acting on a single object or system. When you have exactly two equal and opposite forces acting on the same system, the net force is zero, and the system will not accelerate. But the two forces implied by Newton's 3rd Law are acting on different things. 

It's have to be 12 or idk